The present invention described herein relates to GaAs semiconductor devices and, more particularly, to field effect transistors (FET's) whose gates are self-aligned to the source and the drain.
In the prior art, a GaAs FET is made by a process which starts out by etching of mesa islands down to an undoped GaAs layer adjacent to a semi-insulating substrate to localize the active region. The source and drain ohmic contacts are defined and formed with a gold-germanium eutectic alloy and gold overlay. The rectifying contact for the gate is provided by the deposition of aluminum or a refractory on the surface of the undoped GaAs of the top epilayer. The disadvantages of this method are: (1) the gate may not be properly aligned with the source and drain; (2) source-drain sintering must be accomplished prior to gate definition; and (3) each gate must be individually defined.
Presently, selectively doped GaAs/AlGaAs heterojunction structures are being utilized for high performance digital applications. This so because the selectively doped GaAs/AlGaAs devices provide high electron mobility transistors that exhibit high speed microwave capabilities. However, at present, such selectively doped GaAs/AlGaAs devices are not self-aligned. Yet, in VLSI level circuits, in order to achieve high density, the gate of the FET must be self-aligned to the source and drain. It would be desirable, therefore, to provide a selectively doped GaAs/AlGaAs heterojunction device that has self-aligned gates.
In the prior art, the etching technique known as orientation-dependent etching is known. However, this techniques has not been successfully employed in the making of self-aligned gate GaAs FET's. It would be desirable to be able to employ orientation-dependent etching in making GaAs FET.
In the prior art, FET's having submicron length gates are known. However, fabrication of such devices requires complex photolithographic, optical microscopy based processes. It would be desirable to provide FET's having gates of submicron length without the need for the complex photolithographic based systems.